Version XAMP 1.4.16 with MYSQL 4.1.15
where the column password for the mysql.user table as varchar(16) instead of varchar(41) !
This problem block a connection whis a user/pass that whas crypted with the new style of haching password !

In the new version 1.5.5 , the same problem is here ...
the column password of the user table is set with varchar(16)
and where the length is set to varchar(41),
bof, no more access to phpmyadmin to put the password of root or another user with the new style of password ...
i'm not blocked, i replace the mysql directory by an old installed from another package and go on !!